Electroless metal deposition baths, e.g., electroless copper solutions, are known in the art as useful for providing metal deposits on non-metallic and metallic surfaces. Such solutions are characterized by the capacity to deposit or plate metal in virtually any desired thickness without the need for supplying electrons from an external source of current. After an electroless metal deposit is formed on the surface of the article, the electroless solution becomes autocatalytic, i.e., continues to deposit metal so long as the solution is replenished and maintained.
Special mention is made of the use of electroless metallizing procedures in the plating of plastics generally, and in the manufacture of printed circuit boards particularly. In the plating of plastics, copper is electrolessly deposited on the surface of an article to produce a metallized plastic article for use, e.g., in the auto industry as grills or decorative stripping, in the toy industry as metallized plastic miniatures, and in home products as door knobs and the like.
In the manufacture of printed circuit boards, in one type of procedure a metallic or sensitized non-metallic substratum is provided with copper foil or a thin layer of electrolessly deposited copper. Selected areas of the copper corresponding to the desired circuit pattern are then masked. The unmasked copper is then build up, as by electroplating. The masking material is removed from the copper. The thin layer of background copper is next etched away. In another type of procedure, the surface of the substratum is sensitized for electroless deposition, areas not corresponding to the circuit pattern are masked, and copper is deposited on the unmasked sensitized areas by electroless and/or electroplating techniques until the desired copper thickness is attained.
When electroless plating operations of the foregoing type are practiced on a commercial scale, typically, large plating vessels are employed. The parts or articles to be plated are immersed in the electroless copper deposition solution in the plating vessel. The parts or articles are, moreover, supported on racks or frames immersed in the deposition solution. It has been found, in practice, that if the plating vessel and support racks are constructed of a non-metal, e.g., plastic or glass, copper particles forming in the electroless copper solution during the electroless plating reaction become attached to such equipment surfaces. The attached copper particles provide sites for further electroless copper deposition and growth. If this phenomenon is permitted to proceed unimpeded, eventually most or substantially all of such equipment surfaces become covered with a deposit of electroless copper. This weakens the deposition solution. Furthermore, the plating operation must be interrupted to empty the tank and etch away the copper from the non-metallic equipment surfaces. This etching procedure, though necessary to recapture the copper and avoid waste, is nevertheless disadvantageous in that the plating operation must be stopped, etching chemicals must be stored and handled, etc., and large amounts of expensive etching chemicals are often required. Thus, additional costs are imposed due to interruptions in the plating operation, and because of the labor involved in the etching process. A further disadvantage is that this etching weakens the structure of the non-metallic plating vessels, racks, etc. and shortens their useful life.
The use of plating vessels and other plating equipment made of metallic material is desirable because of greater durability and normally greater availability. These are nevertheless subject to many of the same disadvantages just described. Specifically, copper deposits form continuously and catalytically on the metallic equipment surfaces in contact with the electroless deposition solution, and plating occurs even faster than in those cases where non-metallic equipment is employed. This problem is particularly acute where adjustable metal racks, i.e., containing metal fasteners, are used because the fasteners become coated with an electroless metal deposit making difficult their release.
It is known in the art that plating vessels made of metallic retaining walls, as well as other metallic plating equipment such as racks, plumbing and the like, can be rendered temporarily resistant to electroless metal deposition by pretreatment with chemicals, e.g., nitric acid solutions. Such chemical treatments tend to wear off within hours of operation, however. Consequently, the plating procedure must be interrupted to empty the plating vessel and renew the chemical treatment.
Klein et al., in U.S. Pat. No. 3,424,660, disclose that plating vessels having metallic retaining walls can be protected against electroless metal deposition, particularly electroless nickel, by imposing a potential thereon at a value corresponding to the rest potential or the protection potential range on the current density/potential curve. The current density is adjusted to not more than about 10.sup.-4 amperes per square centimeter.
German Offenlegungsschrift No. 2639247 discloses that plating tanks and racks made of a metal such as cobalt or nickel can be rendered resistant to electroless metal deposition, such as electroless copper deposition, by charging with a current density of at least 4 milliamperes per square decimeter.
It has also been proposed, in Japan Patent Publication No. 54-36577, dated Nov. 9, 1979, that a metal plating vessel, such as of chromium-nickel steel, can be rendered resistant to chemical plating if a positive electrical potential is applied to the plating vessel surface during the plating operation.
In commercial practice, procedures such as the foregoing have not proved satisfactory when applied to electroless copper plating processes. As the electroless copper deposition reaction proceeds, plating chemicals in the solution must be replenished. The chemical replenishment usually causes local fluctuations in the concentrations of chemicals and the introduction of impurities into the solution. Then too, dirt or dust gradually accumulates in the deposition solution. As a result, discrete particles of precipitated copper form in the deposition solution and come into contact with the equipment surfaces. Such copper precipitates draw high current through the metallic plating equipment in prior art processes, causing the electrical potential imposed on such equipment to decay and fall below that needed to resist electroless deposits. An equivalent area of metallic copper requires at least two orders of magnitude more current than stainless steel to resist electroless copper deposition. Thus, although the equipment surfaces in prior art processes are resistant to electroless deposition during the initial stages of the plating operation, when attempts are made to operate such prior art processes on a commercial scale, with replenishment and unavoidable copper precipitation from the bulk solution, after a brief period the equipment surfaces lose their resistance. Thus, prior art procedures have not been successfully adapted to commercial use.
An additional problem is often encountered in the manufacture of printed circuit boards, in particular. In such manufacturing procedures, sometimes layers of copper which are not part of the circuit pattern itself are formed on the borders of the insulating substratum. If such copper borders contact the metal racks supporting the substratum, the current is drastically increased, thus diminishing or decaying the electrical potential applied to the rack such that it eventually falls below the minimum potential required to avoid plating. The prior art procedures have failed to avoid this problem also. The potentiostats of the prior art processes are capable of maximum currents of no greater than one ampere, indicating a lack of knowledge or understanding of the problems encountered in plating on a larger scale, such as in commercial operations.
It is an object of this invention to provide a process for electroless copper deposition in which metallic plating equipment in contact with the deposition solution is rendered initially resistant to electroless copper deposits and maintained in a resistant state for extended periods of the plating operation.
It is another object of this invention to enable use in commercial scale deposition processes of metallic plating equipment for longer periods without the build-up of copper deposits which must be removed by etching.
It is another object of this invention to provide electroless copper deposition processes in which copper precipitates in contact with the plating equipment can be easily removed, as by brushing, sweeping, vacuuming, and the like, without the stopping or shutting down of the plating operation.
It is another object of this invention to provide improved methods of printed circuit board manufacture in which adherent electroless copper deposits on equipment surfaces are avoided.
It is another object of this invention to provide a method for the production of printed circuit boards in which undesired build-ups of electroless copper on the edges and borders of panels being plated is prevented.
The foregoing objects as well as additional objects which will be clear from the following description are achieved by the process of the invention now described.
Notably, the practice of this invention can result in a reduction in the cost of plating of up to 30 percent or more, due principally to savings on plating chemicals and acids and neutralizing bases needed to periodically etch copper away from plating tanks and other equipment surfaces. Additional savings are achieved due to avoidance of labor costs and lost production time necessitated by such etching procedures.